1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of driving a liquid crystal display device with low power consumption.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices display moving images using thin film transistors (TFTs) as a switching element. LCD devices have been widely used for portable applications because of their small size and light weight as compared with cathode ray tubes (CRTs).
FIG. 1 is an equivalent circuit diagram of a liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display (LCD) device 10 includes a timing controller 30, a gray level voltage generator 20, a gate driver 40, a data driver 50 and a liquid crystal panel 60. The timing controller 30 generates several signals for displaying images using a video signal and a synchronous signal of a central process unit (not shown) and supplies the several signals to the gate driver 40 and the data driver 50. The gray level voltage generator 20 provides gray level voltages “V1” to “Vi” corresponding to i-gray levels to the data driver 50. For example, when a color data has an 8-bit format, the gray level voltage generator 20 generates gray level voltages “V1” to “V256” corresponding to 256-gray levels of 28 driver 40 drives a gate line according to the signal of the timing controller 30 and the data driver 50 drives a data line according to the signal of the timing controller 30.
The liquid crystal panel 60 includes the gate line and the data line crossing each other to define a pixel region. A thin film transistor (TFT) “T” is connected to the gate line and the data line. A liquid crystal capacitor “CLC” and a storage capacitor “CST” are connected to the TFT “T.” A gate electrode and a source electrode of the TFT “T” are connected to the gate line and the data line, respectively. A pixel electrode (not shown) is connected to a drain electrode of the TFT “T” and the liquid crystal capacitor “CST” is disposed between the pixel electrode and a common electrode (not shown).
For one frame, gate lines are sequentially selected by the gate driver 40 and a gate signal is supplied to the selected gate line. When the gate signal is supplied to the gate electrode of the TFT “T,” the TFT “T” is turned on and a channel is generated. In addition, a data signal according to image information is supplied to the data line from the data driver 50 and charges up the liquid crystal capacitor “CLC” and the storage capacitor “CST” through the TFT “T.” After the TFT “T” is turned off, the data signal applied to the liquid crystal capacitor “CLC” and the storage capacitor “CST” is kept. Especially, a voltage of the pixel electrode is kept until the next frame by the storage capacitor “CST.”
The LCD device 10 displays images by modulating liquid crystal molecules according to the data signal applied to the liquid crystal capacitor “CLC” and the storage capacitor “CST.” If the data signal modulating the liquid crystal molecules has the same polarity through the frames, the liquid crystal molecules may deteriorate, thereby degrading the display quality. The problems of deterioration of the liquid crystal molecules can be solved by a data inversion driving method, where the data signal has an opposite polarity in every frame.
The data inversion driving method may be classified into a line inversion method, a column inversion method, or a dot inventions method. In a line inversion driving method, the data signals having a positive (+) polarity and a negative (−) polarity are alternately supplied according to the gate line. Accordingly, a voltage of a pixel electrode connected to an even gate line has an opposite polarity to a voltage of a pixel electrode connected to an odd gate line. In a column inversion driving method, the data signals having a positive (+) polarity and a negative (−) polarity are alternately supplied according to the data line. Accordingly, a voltage of a pixel electrode connected to an even data line has an opposite polarity to a voltage of a pixel electrode connected to an odd data line. In a dot inversion driving method, the data signals are supplied such that voltages of the adjacent pixel electrodes along a horizontal direction and a vertical direction have opposite polarities to each other. A dot inversion driving method is combination of a line inversion driving method and a column inversion driving method. Among these data inversion driving methods, the dot inversion driving method is widely used because of its superior display quality and minimization of flicker.
FIGS. 2A and 2B are schematic views showing a polarity of a pixel electrode in adjacent two frames when a liquid crystal display device is driven by a dot inversion driving method according to the related art. As shown in FIGS. 2A and 2B, a pixel electrode having a positive (+) polarity in a frame has a negative (−) polarity in the next frame, and vice versa. In addition, adjacent pixel electrodes have opposite polarities to each other in each frame. When an LCD device is driven by a dot inversion driving method, a common voltage of a fixed value is supplied to a common electrode. Accordingly, a data driver 50 (of FIG. 1) alternately outputs data signals having a positive (+) polarity and a negative (−) polarity with the common voltage as a central value in every frame.
FIG. 3 is an equivalent circuit diagram of a liquid crystal display device driven by a dot inversion driving method according to the related art, and FIG. 4 is a timing chart showing waveforms of a data signal output from a data driver of FIG. 2.
In FIG. 3, a liquid crystal panel 60 includes a data line resistor “RL” and a data line capacitor “CL.” The data line resistor “RL” represents a substantial resistor of a data line and the data line capacitor “CL” represents a total parasitic capacitor between a data line and an adjacent data line and between a data line and a gate line. Although not shown in FIG. 2, a storage capacitor “CST” is connected to the data line capacitor “CL” through a TFT when a gate signal is supplied to the TFT through a gate line. A data driver 50 is connected to the liquid crystal panel 60 through a data line “DL1” to “DLm+2.”
In FIG. 4, a data signal has upper and lower waveforms with respect to a common voltage “VCOM” such that a pixel electrode has one of positive (+) and negative (−) polarities. When a data signal higher than the common voltage “VCOM” is supplied to an mth data line “DLm,” a data signal lower than the common voltage “VCOM” is supplied to an (m+1)th data line “DLm+1.” Accordingly, adjacent pixel electrodes connected the same gate line have opposite polarities to each other. In addition, if a data signal higher than the common voltage “VCOM” is supplied to a pixel electrode connected to an nth gate line, a data signal lower than the common voltage “VCOM” is supplied to a pixel electrode connected to an (n+1)th gate line. As a result, adjacent pixel electrodes of the liquid crystal panel 60 (of FIG. 2) along a horizontal direction and a vertical direction have opposite polarities to each other.
Referring to FIGS. 3 and 4, a first time period “t1,” represents a driving time periods such that a gate signal is supplied to an nth gate line from the gate driver and a second time period “t2” represents a driving time period such that a gate signal is supplied to an (n+1)th gate line from the gate driver. A data driver 50 outputs data signals. When a gate signal is supplied to an nth gate line during the first time period “t1,” a data signal of VCOM−VS/2 is supplied to an mth data line “DLm” and a data signal of VCOM+VS/2 is supplied to an (m+1)th data line “DLm+1, ” Next, when a gate signal is supplied to an (n+1)th gate line during the second time period “t2,” a data signal of VCOM+VS/2 is supplied to an mth data line “DLm” and a data signal of VCOM−VS/2 is supplied to an (m+1)th data line “DLm+1,” Accordingly, a data signal swing is obtained from difference between two data signals such that (VCOM+VS/2)−(VCOM−VS/2)=VS. Since the data driver 50 outputs a data signal having a swing of about VS whenever the gate signal is supplied to the gate line, an LCD device driven by a dot inversion driving method has a high power consumption.
FIG. 5 is an equivalent circuit diagram of a liquid crystal display device having a charge-sharing unit according to the related art. In FIG. 5, a charge-sharing unit 170 including an amplifier “AMP” and a switch “SW” is disposed between a data driver 150 and a liquid crystal panel 160. The amplifier “AMP” and the switch “SW” are connected to an amplifier control terminal “A” and a switch control terminal “B,” respectively. The amplifier “AMP” amplifies a data signal output from the data driver 150. In addition, the switch “SW” connects adjacent data lines “DL1” to “DLm+2” for a predetermined time period, thereby sharing charges of the adjacent data lines “DL1” to “DLm+2.”
FIG. 6 is a timing chart showing a data signal output from a data driver and a switch control signal of a switch control terminal of FIG. 5. In FIG. 6, a first time period “t1” represents a driving time periods such that a gate signal is supplied to an nth gate line from the gate driver and a second time period “t2” represents a driving time period such that a gate signal is supplied to an (n+1)th gate line from the gate driver. In addition, a third time period “t3” and a fourth time period “t4” represent partial time periods of the first and second time periods such that a switch control signal is supplied to a switch control terminal “B” to turn on a switch “SW,” respectively. In other words, the third and fourth time period represent a pre-charging time period or a charge-sharing time period.
Referring to FIGS. 5 and 6, when a gate signal is supplied to an nth gate line during the first time period “t1,” a data signal of VCOM+VS/2 is supplied to an (m+1)th data line “DLm+1” and a data signal of VCOM−VS/2 is supplied to an (m+2)th data line “DLm+2” to charge up a data line capacitor “CL.” Next, when a gate signal is supplied to an (n+1)th gate line during the second time period “t2,” a switch control signal is supplied to the switch control terminal “B” during the fourth time period “t4” to turn on the switch “SW.” Accordingly, the (m+1)th data line “DLm+1” and the (m+2)th data line “DLm+2” are connected to each other for pre-charging. When the switch “SW” is turned on, the (m+1)th data line “DLm+1” and the (m+2)th data line “DLm+2” are connected to each other in parallel. Thus, the data line capacitors “CL” share charges with each other, thereby having the common voltage “VCOM” instantaneously. During the fourth time period “t4,” since the gate signal is supplied to the (n+1)th gate line, the data line capacitors “CL” share charges with the storage capacitor “CST” connected to the (n+1)th gate line.
In general, since capacitance of the data line capacitors “CL” is about 50 times as large as that of the storage capacitor “CST,” the storage capacitor “CST” has approximately the common voltage “VCOM” of the data line capacitors “CL.” Therefore, during the fourth time period “t4,” the data line capacitors “CL” and the storage capacitor “CST” share charges with each other, and therefore, the storage capacitor “CST” approximately has the common voltage “VCOM.” Next, the data driver 150 outputs a data signal of VCOM−VS/2 to the (m+1)th data line “DLm+1” and a data signal of VCOM+VS/2 to the (m+2)th data line “DLm+2.” Since the data lines “DL1” to “DLm+2” have a value of the common voltage “VCOM” before the data signal is output, a data signal swing is obtained from difference between the data signal and the common voltage such that VCOM−(VCOM−VS/2)=VS/2 or VCOM−(VCOM+VS/2)=−VS/2.
In an LCD device having a charge-sharing unit 170, since the data driver 150 outputs a data signal having a swing of about VS/2 for changing polarity of the pixel electrode, the LCD device may be driven by a dot inversion driving method with a relatively low power consumption. However, an additional external driving circuit is required to supply a common voltage “VCOM” to a data line capacitor “CL.” The additional external driving circuit makes the fabrication process complicated and causes increase of production cost.